The subject matter herein relates generally to electric vehicle support equipment having a smart plug with a control circuit for operating a relay.
Relays are used in different types of applications. One application is an automotive application for connecting an electrical vehicle to an external power source. In operation, the relay is activated to enable a charging current to be delivered to the electrical vehicle. The relay is then deactivated to prevent the charging current from being delivered to the electrical vehicle.
In circuits that convey relatively high levels of current, such as the circuits utilized to charge electric vehicles, electric arcs may be generated inside the relay when the contacts within the relay are repositioned from the de-activated state, referred to herein as breaking the contacts, to the activated state, referred to herein as making the contacts. Because the relay is an electromechanical device, the relay exhibits mechanical behavior during operation. Thus, when the relay is activated, the contacts may temporarily bounce or chatter before the contacts are in the final position, i.e. a make position. While the distance between the contacts during a chatter event may be less than ten micrometers, the residual voltage within the relay creates a relatively large electrical field that results in the arc.
The electric arc may be of relatively high energy. If the arc is of sufficiently high energy, the arc may damage and/or contaminate the contacts in the relay. Moreover, in some instances the arc may cause sufficient heat to be generated to weld the contacts together. For example, the arc may weld the contacts together such that the contacts cannot be separated to open the circuit to which the relay is connected. Mechanical dampening devices may be utilized to reduce the chatter. However, the mechanical dampening devices may not fully eliminate the chatter.
A need remains for a smart plug having a relay control circuit that is operable to activate and deactivate a relay controlling relatively large electric currents while reducing excessive arcing that may cause damage to the contacts in the relay. A need remains for a relay control circuit that provides protection from fault conditions.